Method controlling ink application in a printing press

Abstract

In order to control ink application in a printing press, a sheet (3) printed by the printing press (1) is colorimetrically measured in a number of pixels (4) with respect to a selected color coordinate system that has been expanded to be four-dimensional by also taking into account an infrared component. Color difference vectors with respect to the desired color vectors, predefined or determined from a reference sheet (3) and referred to the same color coordinate system, are computed from the color vectors obtained for each pixel (4). A sensitivity matrix is determined for each measured pixel (4) of the sheet (3). The pixels (4) are classified by sensitivity class. The color difference vectors and sensitivity matrices of the pixels (4) pertaining to the same sensitivity class are averaged for each sensitivity class, and input parameters, in particular film thickness modification vectors, are computed from the averaged color difference vectors and averaged sensitivity matrices of all sensitivity classes for a control unit (9) of the inking mechanisms of the printing press (1). The inking of the printing press (1) is then controlled on the basis of the input parameters thus computed.

What is claimed is: 
     
       1. A method of controlling an ink application in a printing press, a sheet being printed by the printing press, the method comprising: colorimetrically measuring the sheet in a plurality of pixels with reference to a selected color coordinate system to obtain a color vector for each pixel;   determining a color difference vector from the color vector for each pixel with respect to a reference color vector for the pixel, the reference color vector being previously defined or determined from a reference printed sheet;   determining a sensitivity matrix for each pixel;   classifying each pixel by a plurality of sensitivity classes;   averaging the color difference vector and the sensitivity matrix of each pixel within one of the plurality of sensitivity classes to form an averaged color difference vector and an averaged sensitivity matrix for each of the plurality of sensitivity classes; and   calculating input parameters as a function of the averaged color difference vectors and the averaged sensitivity matrices for the plurality of sensitivity classes, the ink application being controlled as a function of the input parameters.   
     
     
       2. The method as recited in claim 1 wherein the sensitivity matrices are determined from predetermined ink coverage values. 
     
     
       3. The method as recited in claim 1 wherein the color vector for each pixel includes at least one measured value obtained in the near infrared range, the color vector being four-dimensional with three components of the color vector being coordinate values of an approximately equidistant color space and a fourth component being formed from the at least one measured value in the near infrared range so that the color vector has four components, and wherein the color difference vector measured for each pixel is four-dimensional; and the sensitivity matrix determined for each pixel is formed by gradients of the four components. 
     
     
       4. The method as recited in claim 1 wherein the color vector for each pixel includes at least one measured value obtained in the near infrared range, the color vector for each pixel being four-dimensional with three components of the color vector being coordinate values of an approximately equidistant color space and a fourth component being formed from the at least one measured value in the near infrared range and wherein the color difference vector for each pixel is three-dimensional; and the sensitivity matrix determined for each pixel is formed by gradients of the three components. 
     
     
       5. The method as recited in claim 1 wherein the averaging includes a weighted averaging with at least one weighting factor, the at least one weighting factor being a function of at least one of an ink coverage and a color difference of the pixel with respect to at least one neighboring pixel. 
     
     
       6. The method as recited in claim 5 wherein the at least one weighting factor includes a first weighting factor for a respective pixel, the first weighting factor having a value 1 if an average value or one of the ink coverages of the respective pixel exceeds a predefined first threshold value and otherwise the first weighting factor has an other value less than the value 1. 
     
     
       7. The method as recited in claim 6 wherein the other value is zero and the predefined threshold value is ten percent. 
     
     
       8. The method as recited in claim 5 wherein the at least one weighting factor includes a first weighting factor for a respective pixel, the first weighting factor forming a maximum value if a sum of the ink coverage of the respective pixel is less than a predefined threshold and otherwise the weighting factor is an other value less than the value 1. 
     
     
       9. The method as recited in claim 8 wherein the maximum value is one, the other value is zero and the predefined threshold is 250. 
     
     
       10. The method as recited in claim 1 wherein the averaging includes a weighted averaging using at least a first weighting factor, the first weighting factor being a function of a color difference with respect to an unprinted spot of the printed sheet; the first weighting factor equalize a value 1 if the color difference of a respective pixel exceeds a predefined threshold value, and otherwise the first weighting factor obtains an other value less than the value 1. 
     
     
       11. The method as recited in claim 10 wherein the predefined threshold value equals five and the other value equals one. 
     
     
       12. The method as recited in claim 5 wherein for each pixel, color differences with respect to other pixels in an immediate proximity are determined; a second weighting factor of each pixel equaling a value 1 if a sum of the color differences is less than a predefined threshold value, and otherwise the second weighting factor obtains a value approaching zero as the sum of the color differences increases or as a difference in the ink coverages with respect to the other pixels increases. 
     
     
       13. The method as recited in claim 12 wherein the predefined threshold value equals 8. 
     
     
       14. The method as recited in claim 12 wherein the at least one weighting factor includes a linked weighting factor formed as a function of the second weighting factor and a first weighting factor calculated on the basis of the ink coverages or of the color difference of the pixel with respect to an unprinted spot of the sheet. 
     
     
       15. The method as recited in claim 1 wherein determining the sensitivity matrix includes computing a corresponding sensitivity matrix for a predefined number of discrete screen value combinations of inks involved in a printing process and storing the corresponding sensitivity matrix in a screen-color table, computing a corresponding computed screen value combination for each pixel from the color vectors, and assigning the sensitivity matrix whose respective discrete screen value combination is closest to the computed screen value combination to the pixel. 
     
     
       16. The method as recited in claim 15 wherein a second number of discrete color loci is defined in a color space extended to four dimensions by an infrared component, the corresponding computed screen value combination being computed for each of the discrete color loci, the corresponding computed screen value combination being replaced with a closest of the discrete screen value combination for each of the discrete color loci, associations of discrete color loci with the discrete screen value combination being saved in a screen index table. 
     
     
       17. The method as recited in claim 16 wherein the color vector determined for a respective pixel is replaced with a closest discrete color locus; the screen value combination associated with the discrete color locus being taken from the screen index table and the sensitivity matrix being taken from the screen color table. 
     
     
       18. The method as recited in claim 1 wherein the sensitivity matrices are determined as a function of a mathematical model of the printing press, the mathematical model being a function of measured values on full tone areas printed with the printing press and characteristic curves of the printing press. 
     
     
       19. The method as recited in claim 1 wherein the input parameters are film thickness modification vectors for a control unit for inking mechanisms of the printing press.